1. Field of the Invention
The invention relates to glass strengthening methods. More specifically, it relates to chemical treatements of glass for strengthening the microcrack structure on the surface thereof.
2. Description of the Prior Art
Organic fatty acids are known as an element of various mold lubricants in the glass and metal casting arts. The fatty acid is typically reacted with some other chemical and the fatty acid ester is the common reaction product utilized in the mold lubricant. One such recent disclosure in U.S. Pat. No. 3,495,962 to Norton calls for an olefin oxide derivitive of a fatty acid to serve as an emulsifier in a mold lubricant comprising a mixture of oil and water. No known prior art discloses the use of organic fatty acid not in combination with other chemicals as a treatment for glass, and no known prior art discloses the ability or organic fatty acid to strengthen the surface of glass.
Recent efforts toward increasing the durability of glass containers have called for protective coatings of plastic materials. These coatings increase the durability of the glass by providing a cushion against scratching and impact and help to contain glass fragments if a container should shatter, but no coating is known that actually strengthens the glass surface.
While some containers are coated to increase durability, other containers are being manufactured with thinner walls in order to make them lighter and cheaper in cost. Many of these lighter containers are non-returnable bottles, as their light construction render them unfit for reuse. This approach to container manufacture has the disadvantage of generating waste and often being unfeasable because of state laws requiring beverage bottles to be returnable. Thus, a great need exists for a method of strengthening glass while allowing cost saving through the use of thinner glass container walls.
The present invention is intended to strengthen glass by providing a remedy for those defects in glass structure believed to be responsible for the failure of a glass surface under load. A common concept of the reason for glass failure originated over 40 years ago when Griffith outlined his theory of surface defects existing in materials, especially metals. This theory has been accepted as applying to glass in recent years. The Griffith theory postulates that flaws or microcracks are present in all surfaces and that these microcracks produce high stress concentration when under load. Hence, the method of the present invention attempts to use surface chemistry, and in particular an appropriate surface activating agent such as the organic fatty acids, to alter the surface defect sensitivity of glass by chemisorbing the surface active agent on the surfaces or tips of the microcracks.